The analysis of cell types in rat mammary gland based on their growth response to the phases of the estrus cycle will be completed. An analysis of lesions induced by nitrosomethylurea in the rat mammary gland will be carried out, and the cell types involved will be determined. The specificity of monoclonal antibodies prepared to rat and human mammary cells will be studied, in terms both of the cell types to which they bind in vivo and of the cell surface proteins they immunoprecipitate in cultures. New monoclonal antibodies to human mammary cells, both normal and neoplastic, will be isolated. The role of the Thy-1 antigen in the differentiation of rat mammary cells will be studied further. The mechanism by which lipids (fatty acids, lysolecithins) induce dome formation in cultures of rat mammary cells will be pursued.